Combustion processes are often used to heat oxidation sensitive materials in furnaces. For example, burners are used to preheat the scrap steel feed in electric arc furnace steelmaking processes. Practical considerations often require that the burner be placed at a distance from the material to be heated that is much greater than the optimum distance preferred from a purely heat transfer viewpoint. In addition, heating oxidation sensitive materials requires either control of the oxidation potential at the surface or a subsequent reduction step.
U.S. Pat. No. 6,250,915 seeks to address this problem for gaseous fuels by disclosing use of parallel fuel-rich and fuel-lean gaseous jets. Impingement of the parallel fuel-rich and fuel-lean jets on a surface initiates combustion reactions in close proximity to the surface, which substantially increases the heat transfer rate and efficiency. The jet array can be adjusted to maintain appropriate fuel-rich or fuel-lean atmosphere in contact with the surface. In one embodiment, the use of a low velocity coherent jet consisting of an array of parallel fuel-rich and fuel-lean zones provides efficient heat transfer and reasonable control of the oxidation potential of the melting zone. This technique increases the heat transfer rate to the surface and the ability to control surface oxidation. This technique is effective when the fuel-rich and fuel-jets have similar densities, which in turn requires gaseous fuels and oxidants.
There are, however, many industrial processes where gaseous fuels or oxidants are not used because solid fuels, oxidants, and/or reagents are preferred; for example, coal, carbon or coke are usually the preferred reducing agents and fuels for most iron and steel production processes. By way of further example, metallurgical processes typically require the injection of particulate reagents, e.g., lime.
U.S. Pat. No. 6,254,379 seeks to address particulate injection by having a reagent containing carrier gas pass through a flame envelope. The flame envelope forms a fluid shield or barrier around the gas jet to minimize ingression of gas into the jet and maintain a coherent jet. The flame envelope has a velocity that is less than the velocity of the jet. As the jet exits the flame envelope, the rate of gas entrainment increases and the jet loses its coherency and delivers the reagent to a diffuse reaction zone as a turbulent jet. In contrast, the heating method disclosed in U.S. Pat. No. 6,250,915 teaches that the jet coherency be maintained, to the maximum extent possible, until the jet impacts the surface in order to provide a well-defined reaction zone and increase heat and mass transfer efficiency to the surface. Metallurgical processes typically require significant heat input coincident with solids injection.
U.S. Pat. No. 5,954,855 discloses techniques to use direct flame impingement, high velocity oxygen jets, and carbonaceous fuel injection jets to melt steel in electric arc furnaces. High efficiency melting requires the simultaneous feeding of oxygen and a carbonaceous jet to the melting zone. However, it is very difficult to precisely feed oxygen and carbon from separate lances, with different characteristics, to a melting zone that is a constantly moving melting steel surface.
Despite the above teachings, there is no process disclosed for condensed phase fuels, reducing, or oxidizing agents. Therefore, there is still a need for a system to generate a coherent jet having a fuel-rich zone, a fuel-lean zone, and a condensed phase fuel or reagent to efficiently heat and treat surfaces. It would also be desirable to have a system whereby troublesome by-product iron oxide fines from metallurgical processes could be recycled using an efficient particulate injection and heating method.